The role of calcium in initiating smooth muscle contraction is widely documented. The intracellular sinks and sources of activator calcium have not been clearly delineated although SR, mitochondria and plasma membrane have been suggested as possible sites of calcium sequestration. It has been previously proposed that spasmolytic agents exert at least part of their relaxing effect by altering normal calcium movements into calcium sinks and/or out of the calcium sources. In this grant proposal a method is presented with which the distribution of calcium among intracellular structures can be determined. This method involves the electron microscopic examination of freeze-dried, smooth muscle that has been prepared for 45Ca autoradiography. By employing these techniques, we propose to examine the effects of smooth muscle spasmogenic and spasmolytic agents on the distribution of calcium among intracellular structures. We anticipate that an analysis of these data will help us to more clearly understand how these pharmacological agents exert their physiological effects on smooth muscle.